Field of the Invention
The present invention relates to a technique for measuring concentration of a target substance in liquid, for example, concentration of toner in liquid developer.
Description of the Related Art
There is a conventional concentration measurement device for measuring concentration of a target substance in liquid. For example, an electrophotographic apparatus is provided with a concentration measurement device that measures toner concentration in liquid developer including toner particles and carrier liquid for distributing the toner particles. A liquid development device in an electrophotographic apparatus develops an electrostatic latent image formed on an image bearing member using liquid developer. Value of liquid developer is being revised in recent years because of an advantage that is unrealizable with dry powders developer. Since an electrophotographic apparatus uses extremely minute toner of submicron size, high definition is achieved and texture of a printing press level is obtained. Moreover, since toner in liquid developer is fixed to a paper sheet at a relatively low temperature, power consumption is saved.
Such a liquid development device may develop an image after pumping up liquid developer from a liquid-developer tank once in order to prevent carrier liquid from adhering excessively to a surface of an image bearing member. For example, a part of a developing roller is immersed in the liquid developer in the liquid-developer tank, and the image is developed by bringing the liquid developer adhered by rotating the developing roller into contact with the image bearing member. Such a development device is needed to control a toner concentration in carrier liquid within a predetermined range in order to maintain image density on a paper sheet uniformly and to maintain high definition. This needs control that measures the toner concentration in the liquid developer accurately and keeps the concentration constant.
For example, there is a proposed concentration measurement device that detects and measures toner concentration on the basis of an optical transmittance of liquid developer after giving external pressure to a part of pipe through which the liquid developer is sent to thin thickness of the part (Japanese Laid-Open Patent Publication (Kokai) No. 2009-175386 (JP 2009-175386A)). Since the device measures the toner concentration in a state where the liquid developer flows, precipitation of the toner is prevented, which achieves the concentration measurement at a high accuracy. However, bubbles may generate in the liquid developer sent to the concentration measurement device due to various factors, such as mixing from a gap of a coupling of a pipe. The bubbles may change the optical transmittance and cause a measurement error.
Japanese Laid-Open Patent Publication (Kokai) No. H6-314031 (JP H6-314031A) proposes a configuration that branches a pipe that sends liquid into a measurement path and a bypassing path arranged above the measurement path. A toner concentration measurement device is arranged in the measurement path. Most of bubbles flow through the bypassing path due to buoyant force. Accordingly, the configuration prevents degradation of measurement accuracy of the toner concentration due to bubbles.
The configuration of JP H6-314031A reduces mixing of bubbles to the liquid developer but cannot detect presence or absence of bubbles. As mentioned above, the measurement error becomes large in a case where the toner concentration of the liquid developer is measured in a state where bubbles are mixed. Moreover, when an image is formed by the liquid developer in which the bubbles are mixed, a liquid-layer unformed line may occur on a developing roller. In that case, an image defect may occur because a toner untransferred region may occur in an image that is finally formed on a paper sheet through the following image forming process. It should be noted that installation of an exclusive configuration for determining the presence or absence of bubbles raises a cost.